1. Field of the Invention:
This invention relates generally to a protective relay apparatus of the distance type, and more specifically, to an improved distance protective relay providing correction for interaction of fault resistance and power-system operating conditions.
2. Description of the Prior Art:
Three-phase ac electrical power transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits or drops in shunt resistance that could cause collapse of the power system and render serious and expensive apparatus damage. For instance, such a fault condition is caused by lightning-induced flashover from a transmission line to ground or between adjacent transmission line conductors. Under such a faulted condition, line currents can increase to several times their normal value, causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment. To avoid equipment damage and collapse of the entire power system, faulted apparatus on the main transmission line must be isolated from the network within a short time interval, say 0.1 to 0.5 seconds, for example. The isolation time limit must allow for the operation of large circuit breakers interrupting up to 80,000 A and the completion of backup operations if these primary protective devices fail to function properly. To allow sufficient time for circuit interruption, location of the fault must be determined in approximately 8 ms to 20 ms. It is the function of the protective relays, which continuously monitor power system ac voltages and currents, to locate line faults and initiate isolation via tripping of the appropriate circuit breakers.
A distance relay is one type of protective relay used by the utility industry to protect the electrical power system. Basically, a distance relay measures the current and voltage of the power system at an end point of a transmission line to determine whether a fault exists inside or outside the protection zone of the relay. The distance relay determines the distance to the fault by calculating the line impedance, based on the measured current and voltage at the line end point. For a fault at the remote end of the protected section of a transmission line, the impedance seen by the protective relay at the local or measuring end is V/I=Z, where Z is the line impedance. For an internal fault on the protected section of the line, V/I&lt;Z. For fault beyond the protected section, V/I&gt;Z. Since Z is proportional to the line length between the protective relay and the fault, it is also a measure of the distance to the fault. Calculating the line impedance determines the distance to the fault. If the fault is on the protected line section or segment, the protective relay trips the appropriate circuit breakers.
Inaccuracies in the determination of the impedance due to various power system phenomena which have an effect on the voltage and current at the protective relay, can cause improper operation of distance protective relays. For example, if a fault occurs outside of the protected line segment, but the impedance determination by the protective relay indicates that the fault is on the protected line segment, the circuit breaker trips but the fault is not cleared. This is an example of overreaching of a distance protective relay. Under other circumstances, the protective relay can also underreach, i.e., not detect a fault located on the protected line segment. One cause of overreaching or underreaching, is the failure to consider residual current in the faulted circuit; another is the failure to consider the residual current in a parallel, unfaulted circuit which is magnetically coupled to the protected circuit. To overcome these problems, the protective relays must compensate for the effects of the residual current in the protected circuit (residual compensation) and in unfaulted parallel circuits (mutual compensation).
Another cause of overreaching and underreaching is the interaction of fault resistance with power-system operating conditions related to load flow prior to the fault. During normal (unfaulted) operating conditions, a distance relay measures an impedance different than that of the transmission line because it also measures the impedance of the load, and is influenced by the effect of other power sources beyond the remote but which feeds the load. If a fault occurs which is a dead short-circuit (zero impedance between or among faulted conductors), the relay measures only the impedance of the line from the relay location to the fault. However, if the fault is not a dead short-circuit, the fault impedance is added (as a complex phasor quantity) to that of the line impedance. For actual faults, the fault path itself is mainly resistive and does not necessarily produce serious errors in the reach measurement, which emphasizes inductive reactance of the line conductors.
Problems arise when power sources are connected at both ends of the line, which is normally the case in extra high voltage (EHV) transmission networks. If the fault has zero impedance, the power sources at the two ends of the line feed the fault independently, and fault current infeed from the remote terminal has no effect on the local distance relay. If the fault path has significant impedance, however, it will produce a voltage drop which is related to total current infeed from both sources. Thus, the remote current infeed can modify the impedance measured by the local relay.
If the remote current infeed is in phase with the local contribution, the voltage across the fault resistance becomes larger than it would be without the remote current. Thus, the fault appears to the local relay to have a larger resistance than it has in reality. If the tripping is based largely on reactance, the magnification of resistance will not necessarily cause a reach error.
If load was flowing in the protected transmission line prior to the fault, the sources at the two ends will have a phase difference which is proportional to the amount of load, and to the source and line impedances. This phase difference is maintained throughout the early stages of a fault. The phase difference of the remote current infeed produces a shift in the phase of the voltage drop across the fault resistance. This introduces the appearance of a reactive element of fault path impedance, even though no reactance is physically present in the fault. This reactance, in turn, causes serious errors in the location of the fault by the local relay. The relay overreaches or underreaches depending on whether the local source current leads or lags the remote infeed. The errors grow in direct proportion to the phase difference and the actual physical resistance of the fault path.
An article entitled "New Distance Protective Relay with Improved Coverage for High-Resistance Earth Faults" by A. T. Johns, and A. A. El-Alaily, appearing in the IEE Proceedings, Vol. 124, No. 4, April 1977, discloses a new protective relay of the distance relay type to improve relay operation for high-resistance earth faults. The protective relay described in this article appears to use a comparison method to compare three derived signals based on the voltages, currents, and impedances of the protected line segment. The boundary characteristic of this protective relay is automatically adjusted based on the angle of the positive-phase-sequence line impedance. The three derived signals are compared and tripping initiated when three arithmetic quantities derived from these three derived signals all lie within the limits of the boundary characteristic.